Acknowledgement

This project has helped us to get acquainted with the cosmic world of Electrical Engineering.

Alongside the sensation of emergent quest, there were incessant grimaces, screw ups, mental

loops, success delights and fantasies in the midst of project undertaking. The moment was the

one to treasure. The joy of having the project done and the feeling of “Yes! We did it” valued our

hard work.

We take this opportunity to express our profound gratitude and deep regards to our Department

of Electrical Engineering for providing us with this project. We are privileged to have Mr.

Mahummad Badrudoza and Mr. Hari Prasad Rimal for their exemplary guidance, monitoring and

constant encouragement throughout the course of this project.

We also take this opportunity to express a deep sense of gratitude to Prof. Dr. Indraman

Tamrakar for his cordial support, valuable information and guidance, which helped us in

completing this task through various stages.

We are obliged to Anil, Shiva, Surendra, and Saket for the valuable information provided by

them in their respective fields. We are grateful for their cooperation during the period of our

project.

Lastly, we thank all who supported us directly or indirectly during the project without which this

project would have been impossible.

Anil Panjiyar

Bijay Paudel

Rabin Shrestha

Rohit Agrawal

ABSTRACT

This project “Design and Fabrication of PWM rectifier to Improve Input Power Factor” deals

with the development of a controlled rectifier for varied applications, the input p.f. of which is

maintained unity. Moreover, one of the important advantages of using PWM technique is the low

value of small order harmonics and large high order harmonics that can be filtered easily.

In our project, we have implemented a digital controller using PIC microcontroller which is used

to generate synchronized control signals, the signals are amplified and are conditioned as per the

gate drive requirements of GTO thyristor.

The project circuitry includes the control circuit (AVR Atmega16), the gate drive unit and GTO

rectifier. The simulations regarding the PIC microcontroller and gate signal generation were

done in ‘Proteus’ whereas the response of the system was studied on ‘MATLAB via. Simulink

based simulation’. As an additional feature we have provided the system with input switching

through which we can operate the rectifier in SPWM and Symmetric mode as well. A

comparative study of the system was done by comparing three methods.

LIST OF FIGURES

LIST OF ABBREVIATIONS

TABLE OF CONTENTS

CHAPTER 1

INTRODUCTION

1.1 Overview of the Project

AC power source is dominant over DC source in industrial as well as other applications due to

it’s flexibility in generation, transmission and distribution. However, DC machines are suited for

some applications than AC machines. In the context, rectifiers are used to convert available AC

power to DC power supply.

Diodes can be used to make rectifiers where we need fixed output but to make controlled

rectifiers thyristors or GTO are used. Phase controlled rectifiers are used in many applications

like Motor Control, Switched Mode Power Supplies etc.

In any electrical system p.f. is a critical issue, similar is the case in PCR. At present time where

effective and efficient sysem design is mandatory, the choice of correct alternative is essential.

Keeping in mind the various advantages of PWM technique to improve input p.f., we are

embarking to develop a rectification system employing this technique.

1.2 Literature Review We have studied several books and papers regarding the use of rectifiers, power factor improvement,

concept of project and components to be used.

1. Muhammad M. Rashid, ‘Power electronics Handbook’

We studied about the rectifiers and phase controlled rectifiers. The study helped to realize the

necessity of the project.

2. P.S. Bhimbra, ‘Power electronics’

This book is written in simple language and it facilitated system analysis. Waveform analysis,

Fourier analysis, Mathematical calculation was done with the help this book.

3. Sedra and Smith, ‘Microelectronic circuit’

Electronics circuits that will be implemented were studied from the book. The studies cover the

use of transistor switches, operational amplifiers, and waveform generations.

4. GTO datasheet for ‘Philips BTW58-1500R’

GTO is the device we will be using so the detail information of the device is compulsory. This

helps in the design of gate drive circuit of GTO. The features of the GTO are:

Fast Turn-Off With Reverse Gate Pulse

High Voltage= 1000 and 1200 volts

Momentarily Forward Pulse For Turn On

Minimizes Drive Losses

High Surge Capability

5. AVR Atmega handbook/ Datasheet

We are using the AVR in order to generate the synchronized control pulses so the skills relating

to use of controller was required. Hence, we studied the books related to microcontroller and its

programming.

1.3 Objectives The primary objective of the project is to ‘Design and fabricate PWM rectifier to improve input

power factor’.

1.4 Methodology

The method adopted for the advancement of project activities can be explained under

following headings:-

Study of Related Theory

First of all we studied the theories related to the project. We consulted different books

to know about rectifiers, controlled rectifiers, p.f. improvement requirement and

PWM control method to improve input power factor. From his study we confirm

system circuitry and its analysis.

Matlab and Proteus Simulation

Modeling of different components in Simulink. Learn and practice different Simulink models and be familiar with tools and

techniques required.

Design Simulink model for PWM rectifier.

Electronic circuit design for control signal generation.

Programming AVR Atmega16.

Testing in Proteus.

Fabrication of the proposed system

The system incorporating the control circuit was designed and fabricated in PCB.

Modular design was preferred.

The control circuit was interfaced with the rectifier unit to get the desired outputs.

Testing in lab

The system outputs for the control circuits and rectifier were viewed by the help of

oscilloscope.

The detailed information will be provided on the following chapters.

CHAPTER 2

THEORITICAL STUDY

2.1 Theoretical Study

Rectifiers and Controlled Rectifiers

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses

direction, to direct current (DC), which flows in only one direction. The process is known as rectification.

Rectifiers have many uses, but are often found serving as components of DC power supplies and high-

voltage direct current power transmission systems.

In a controlled rectifier or a phase-controlled rectifier, the diodes in the rectifier circuit are

replaced by SCRs. These circuits are produced a variable DC output voltage whose magnitude is varied

by phase control, that is, by controlling the duration of the conduction period by varying the point at

which a gate signal is applied to the SCR.

Unlike a diode, an SCR will not automatically conduct when the anode to cathode voltage

becomes positive-a gate pulse must be provided. If we adjust the delay time of the gate pulse, and if this

process is done repeatedly, then the rectifier output can be controlled. This process is called phase control.

Power factor as an important issue

Lets us take a look at single phase semi-convertor shown in the figure below:

Fig: half controlled single phase bridge rectifier

(a) waveforms of input voltage and input current

From the above waveform we see that current lags voltage by some phase angle(α), So that there is

lagging power factor. The PF of phase controlled converters depends on delay angle, and is general low,

especially at the low output voltage range.

Following methods can be implemented to improve input power factor:-

1) Extinction angle control 2) Symmetric angle control 3) PWM control 4) Sinusoidal PWM control

PWM Control Method

In PWM control, the converter switches are turned on and off several times during a half-cycle and the

output voltage is controlled by varying the width of pulses. The gate signals are generated by

programming microcontroller. The lower order harmonics can be eliminated or reduced by selecting the

number of pulses per half-cycle. However, increasing the number of pulses would also increase the

magnitude of higher order harmonics, which could easily be filtered out.

Fig: PWM rectifier

Fig: Output waveform of PWM rectifier

One other advantage of PWM rectifier is low value of lower order harmonics and high value of higher

order harmonics. It is advantageous as the high order harmonics can be filtered out easily.

GTO Gate turn-off (GTO) thyristors are able to not only turn on the main current but also turn it off, provided with a gate drive circuit. Unlike conventional thyristors, they have no commutation circuit, downsizing application systems while improving efficiency. They are the most suitable for high-current, high speed switching applications, such as inverters and chopper circuits. Fig: GTO GTO OPERATION

GTO can be turned on by applying a positive gate signal and can be turned off by applying a negative

signal in its gate. The waveforms can be shown as:

SIMULATION

Matlab Simulation

After learning general Matlab tools, we model our system in Matlab as shown in following diagram:

Fig: Simulink Based Model for PWM rectifier

The output observed after running the simulation is also shown below:

Fig: Output waveform for PWM rectifier

This Matlab simulation works fine, however there’s a drawback. The GTO used in the simulation process

doesn’t require negative pulses to turn off, which is not practical. So, for actual implementation, the gate

drive signal must compose of positive pulse as well as negative pulse. This pulse can be generated by

using a suitable signal conditioning circuit, its matlab model and simulation results shown below:

Fig: GTO gate drive circuit

The output waveforms were observed as:

Fig: Output waveform of GTO gate drive circuit

RC1 and RC2 are signals generated from two pins of microcontrollers. One thing noticed in above

diagram is the magnitude of negative current and voltage being greater than that of positive current and

voltage. This has been made due to the reason that GTO require almost twice more current to turn off

rather than to turn on (as per datasheet). Capacitor has been used for the purpose.

In absence of negative another circuitry (H-bridge) might be implemented, however former one is

preferred due to its simplicity and economic considerations.

PROTEUS SIMULATION

In proteus, we have interfaced PIC microcontroller with zero level detector and input interface

(button/Keypad). Zero crossing detectors comprises of the optocouplers and transformer that performs

dual function of isolation and zero crossing detection. Buttons or Keypads are used so that the conduction

angle of GTO can be varied to get variable voltage output.

The circuit diagram used in proteus is: